DESCRIPTION: (abbreviated from the applicant's page 2) The cellular protein cyclophilin A (CyPA) binds to a proline-rich loop in the CA domain of HIV-1 Gag and is thereby incorporated into virions, where it promotes an early event in the infectious cycle. Inhibitors of the Gag- CyPA interaction disrupt CyPA packaging and attenuate infectivity. The goal of this proposal is to develop a detailed understanding of CyPA function in HIV-1 replication. The four specific aims are: (1) To use human T cells lines engineered to lack functional genes for CyPA, in order to study the ability of HIV-1 to replicate in absence of CyPA, and to test to what extent other cyclophilins can substitute for CyPA. (2) To determine if it is the peptidyl prolyl isomerase (PPIase) activity of CyPA that is the key feature of this protein that performs a function important for the HIV-1 life cycle, or alternatively if it is the CA- binding activity itself. (3) To pinpoint the sequences that determine CyPA dependence in HIV-1, making use of mutants and rare HIV-1 isolates do not need CyPA, even though they incorporate it into virions. (4) To gain biochemical evidence for the function of CyPA for HIV-1 replication, including a test of the hypothesis that this cellular protein acts by destabilizing HIV-1 cores, thereby promoting disassembly in the newly infected cell.